1. Field of the Invention
This invention is in the field of lasers. In particular, it is a circuit for improving the linearity of high power lasers when such lasers are used to transmit high frequency amplitude modulated ("AM") input signals.
2. Description of the Relevant Art
The use of lasers to transmit information over fiber optic waveguides is known. The ability of such laser/fiber optic networks to transmit large amounts of information quickly far exceeds that of conventional electrically conductive media.
Despite the benefits of high speed and high density data transmission, most lasers currently available for use with such networks have a relatively small power output Attempts to develop solid state lasers which operate at higher power output levels have not been completely successful. Although some high power laser are now available, they have significant nonlinearities when operated at these higher power levels. In particular, as the input current level increases, the output light levels do not increase linearly therewith. This defect is known as "rolloff."
Several techniques have been developed which reduce these nonlinearities and distortion. Muka, et al, U.S. Pat. No. 4,672,192, teaches the use of an acoustooptic cell which receives a noisy input laser signal and uses sampling and feedback techniques to reduce the noise. The Muka, et al., system is complicated and expensive, requiring numerous precision optical components.
Shibagaki, et al., U.S. Pat. No. 4,733,398, uses a feedback arrangement to produce an error signal indicating the difference between the input and output signals. An integration device then produces an average value signal from the difference to obtain a control signal, which in turn is used to modulate the laser's input signal. This system is also complex and requires expensive components.
Straus, et al., in U.S. Pat. No. 4,075,474, teaches an optical transmitter which utilizes two matched light emitting diodes to achieve distortion reduction. Straus, et al., modulates the electrical-to-optical converter using a modified signal. The modified signal is obtained using a matched emitting device, presumably a semiconductor laser, which is used to drive an optical receiver. Errors in the received signal versus the driving signal are subtracted from the signal used to control the optical driver which actually emits the signal for the optical fiber. While Straus, et al., is suitable for some applications, it is undesirable for high power applications because of the necessity of acquiring two matched high power lasers. Not only is it extremely difficult to obtain matched high power lasers, the substantial expense involved is highly undesirable.
Straus, et al., in "Linearized Transmitters for Analog Fiber Links," Laser Focus, Oct. 1978, pp. 54-61, describes an optical feed forward system in which the input signal is used to drive an amplifier for the LED. The LED output is monitored and supplied to an error control. The error control also receives the original input signal after an appropriate delay, then compares the two and uses them to drive a correcting LED. A significant disadvantage of the approach employed by Straus, et al., is that the LEDs must be carefully matched. The entire system is predicated on the accuracy of the correcting LED matching the driving LED. Consequently, the minimum cost of the system is twice the price of the LEDs employed. An additional disadvantage is that because Straus, et al., does not tap the fiber to detect the signal on it, the errors corrected cannot match the actual signal as closely as is otherwise desired. A further disadvantage is that Straus, et al., monitors the driving LED by monitoring light which misses the fiber. As power changes, the LED output beam will widen and have a nonuniform power density. Thus, measuring the LED error signal at the input to the fiber does not accurately describe the nature of the signal present within the fiber.
The cited references do not discuss the fact that the electrical input signal to the laser/fiber optic network may be at a level which exceeds the linear operating range of the laser. Making a large reduction in the modulation of the input signal is not effective, as the received modulation would become too noisy.
A need therefore exists for a system which can reduce laser distortion by modifying the electrical input signal to such a laser in an inexpensive and functional manner.